Identification systems are used to locate both animate and inanimate objects. For example, identification systems can locate vehicles carrying interstate freight, employees within a hospital, and items in retail establishments to guard against theft. Identification systems may also be used to locate sources of information. However, in some ways, the difficulty of locating unique information has increased with modern advances in information technology and related storage devices. Because many forms of modern information storage media have a nondescript appearance, there is an increased need to locate specific files and data from among many objects that have similar, or even identical, appearances. For example, a collection of compact discs (CDs) that contain stored data (e.g., digital photographs) may be searched in order to locate the specific CD that contains a particular digital photograph being sought. If the contents of each CD are clearly indicated on the CD or the CD storage case, the search will likely be successful, but it will also be tedious because each CD must be inspected until the photograph is found. If the CD contents are not identified, the search will become even more lengthy because each CD must be inserted into a computer where the contents of the CD may be read. Many other forms of information storage media present similar difficulties in locating specific stored information. As a result, the search for the media containing a specific document or file may become painstakingly tedious because the field of search is large.
The preceding example is directed to a single type of information storage medium, e.g., CDs. However, because more than one type of information storage medium may be suitable for storing a particular type of data, the search for a unique item from among multiple different types of items (e.g., digital audio tapes, floppy disks, DVDs, and miniDVDs) may be required. This multi-medium search presents many of the previously described challenges, particularly when the searcher is not sure which medium contains the unique item that is sought.
These challenges are not limited only to electronic storage media, and information storage media generally. For example, the collection of anonymous boxes and crates found in any warehouse are also difficult to distinguish from one another, and determination of their contents is generally difficult without a close individual investigation.
Many of today's identification systems employ “tags” that may be read by an external (and typically wireless) reader. Generally, identification tags fall into one of two categories: passive and active tags. Passive identification tags are powered by a tag sensing system. For example, the tag circuits of some passive tagging systems are powered via a magnetic, electrostatic or RF field broadcast by a tag reader. Active power tags are those tags that include a power supply. Active tags consume a considerable amount of power in order to receive the tag identification signal, in part because of the linear amplifiers typically required in the receivers of these tags. Because, generally, a battery supplies the power for active tags, these tags have relatively short operational lives before either the battery must be replaced or the tag discarded.
One method that may be employed to conserve the operational life of active tags or other self-powered electronic devices is to provide the device with a lower power “standby” or “sleep” mode. The inherent problem with such a solution is devising a way to “wake up” the device when it is needed. Existing devices require that the device periodically wake itself up in order to ascertain whether or not it has been called, a requirement that necessarily requires power every time the device wakes up. For example, in U.S. Pat. No. 5,387,993 (Heller et al), a tag is disclosed with a “sleep mode”. The “sleep mode” of Heller is a shutdown period of a predetermined time, after which the tag reactivates itself to perform various tasks [Heller at col. 3, lines 48-57]. The sleep mode of Heller is therefore very power-costly, with the tag spending a fair amount of time awake and using power on a fixed, predetermined schedule, even when no transmissions are being sent out by the transmitter. What has been needed, therefore, is a wake-up function for electronic devices that keeps the device in a low-power standby mode until, and unless, an interrogation or other summoning signal is detected.